1. Technical Field of the Invention
The present invention relates generally to a radio communication apparatus including a plurality of sub-transmission units and a main transmission unit controlling communications between the sub-communication units, and more particularly to a radio communication apparatus designed to control standby processing of sub-transmission units waiting for calls.
2. Background Art
As one of radio communication systems, there is a mobile communication system. The mobile communication system consists of a plurality of mobile stations and a base station which monitors and controls communications between the mobile stations. A spread spectrum system is used in the communications for reducing the deterioration of signals due to interference waves and for the security of information.
Regardless of the presence or absence of calls, the base station always transmits signals (referred to as paging signals below) indicative of data on an incoming call (referred to as paging data below) to the mobile stations simultaneously. Upon reception of the paging signal, each mobile station analyzes the paging signal to determine whether the call is given to itself or not.
For example, when one of the mobile stations sends a call request to the base station to call another mobile station, the base station transmits to all the mobile stations the paging signals indicating that the call request has been made. Upon reception of the paging signal, each mobile station analyzes the paging signal to determine whether the incoming call is made to itself or not. One of the mobile stations which has found that the incoming call is made to itself acknowledges to the base station reception of the call. Upon reception of the acknowledgment, the base station allows communication between the calling and called mobile stations.
However, each mobile station in the conventional mobile communication system needs to analyze the paging signal from the base station at all times. The analysis of the paging signal further requires error detection and correction of data contained in the paging signal. The error detection and correction involve complex operations and consumes a large quantity of power. Each mobile station, thus, has the disadvantage that a large quantity of power is consumed at all times in knowing whether a call is made to itself or not.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide an improved radio communication system capable of establishing transmissions of a main transmission unit and sub-transmission units with a decreased power consumption.
According to one aspect of the present invention, there is provided a radio communication system which comprises a plurality of sub-transmission units and a main transmission unit which transmits to the sub-transmission units simultaneously a frame including a data slot to which paging data on a call made by one of the sub-transmission units to another is added and a preamble to which a bit stream to be subjected to double spreading is added for informing the sub-transmission units of a call request. The main transmission unit includes an adding circuit adding to the preamble first data indicating the presence or absence of the paging data, a double spreading circuit subjecting data located in the preamble to the double spreading, and a transmission circuit transmitting the frame to which the first data is added to the sub-transmission units simultaneously. Each of the sub-transmission units includes a receiving circuit receiving the frame from the main unit, an inverse double spreading circuit subjecting the data located in the preamble within the received frame to inverse double spreading, a decision circuit for deciding whether the first data indicates the presence of the paging data or not, a reproducing circuit reproducing the paging data of the received frame, and a switching circuit turning on the reproducing circuit when the first data indicates the presence of the paging data.
In the preferred mode of the invention, the switching circuit keeps the reproducing circuit turned on during a time when the reproducing circuit receives the data slot containing the paging data.
The main transmission unit further includes an amplifying circuit amplifies the frame. An amplification of the data slot within which the paging data is provided is smaller than that of the data slot within which the preamble and the paging data are not provided.
Data for error correction of the paging data is added to the data slot.
The sub-transmission units are classified into a plurality of groups. The adding circuit adds to the preamble second data indicative of one of the groups to which the other sub-transmission units belong. Only the sub-transmission units belonging to the one of the groups indicated by the second data reproduce the paging data.
The adding circuit adds second data specifying the other sub-transmission units to the preamble. Each decision circuit decides which of the sub-transmission units the second data specifies. Only the reproducing circuits in the other sub-transmission units reproduce the paging data.